Internal combustion engines typically include a plurality of pistons which are disposed within a plurality of corresponding combustion cylinders. Each of the pistons is pivotally connected to one end of a piston rod, which in turn is pivotally connected at the other end thereof with a common crankshaft. The relative axial displacement of each piston between a top dead center (TDC) position and a bottom dead center (BDC) position is determined by the angular orientation of the crank arm on the crankshaft with which each piston is connected.
A free piston internal combustion engine likewise includes a plurality of pistons which are reciprocally disposed in a plurality of corresponding combustion cylinders. However, the pistons are not interconnected with each other through the use of a crankshaft. Rather, each piston is typically rigidly connected with a plunger rod which is used to provide some type of work output. In a free piston engine with a hydraulic output, the plunger is used to pump hydraulic fluid which can be used for a particular application. Typically, the housing which defines the combustion cylinder also defines a hydraulic cylinder in which the plunger is disposed and an intermediate compression cylinder between the combustion cylinder and the hydraulic cylinder. The combustion cylinder has the largest inside diameter; the compression cylinder has an inside diameter which is smaller than the combustion cylinder; and the hydraulic cylinder has an inside diameter which is still yet smaller than the compression cylinder. A compression head which is attached to and carried by the plunger at a location between the piston head and plunger head has an outside diameter which is just slightly smaller than the inside diameter of the compression cylinder. A high pressure hydraulic accumulator which is fluidly connected with the hydraulic cylinder is pressurized through the reciprocating movement of the plunger during operation of the free piston engine. An additional hydraulic accumulator is selectively interconnected with the area in the compression cylinder to exert a relatively high axial pressure against the compression head and thereby move the piston head toward the TDC position.
With a free piston engine as described above, the piston will not travel to the original BDC position if a misfire occurs during normal operation or at initial start-up. The piston may not travel a sufficient distance which provides an effective compression ratio for subsequently firing the free piston engine. Upon occurrence of a misfire during initial start-up, the piston may need to be manually returned to a BDC position several times until combustion occurs. Each time the piston is retracted to the BDC position during the manual return operation, the exhaust outlet is uncovered and at least a portion of the non-combusted fuel and air mixture flows to the ambient environment. This results in a loss of energy, especially heat, which was previously imparted to the fuel and air mixture during a previous compression stroke. Moreover, the manual return procedure may take several seconds to complete, which a user may find undesirable.
With conventional free piston internal combustion engines, emissions are a critical issue. Start-up of conventional free piston internal combustion engines is one of the worst operating points for control of emissions.
The present invention is directed to overcoming one or more of the problems as set forth above.